1. Field of the Invention
The present invention relates to an Ohmic electrode used in semiconductor devices, a field effect transistor comprising such an Ohmic electrode, a semiconductor device comprising such an Ohmic electrode, a method of manufacturing the Ohmic electrode, and a method of manufacturing the field effect transistor.
2. Description of Related Art
Gallium nitride semiconductors (hereafter called “GaN semiconductors”) have a high dielectric breakdown voltage and high saturation electron velocity. HEMTs (high-speed mobility transistors) comprising an AlGaN/GaN heterostructure which utilizes these characteristics are attracting attention as power devices capable of high-frequency operation and high withstand-voltage operation to replace GaAs semiconductor devices, for example as high-frequency and high-power amplifier devices in the base stations of cell phone networks.
In general, in order to obtain large output power from the above-described power devices, (1) a large source-drain current, and (2) a high dielectric strength, are effective.
Below, technology of the related art is explained as relating to HEMTs using GaN semiconductors, focusing in particular on the above-described (1) increases in source-drain current.
One method to increase the current between source and drain is to reduce the contact resistance between source and drain electrode and the electron transit layer, described below.
As this type of technology, a technology of the related art in which a depressed portion, called a recess, is provided by dry etching in a region in which source and drain electrodes are to be formed, and the source and drain electrodes are formed in these recesses, is described in a reference (“Recessed Ohmic AlGaN/AlN/GaN HEMTs Grown on 100-mm-diam. Epitaxial AlN/Sapphire Template”, M. MIYOSHI et al, Technical Report of IEICE, ED2004-217, NW2004-224 (2005-01), p. 31-35).
Below, this technology of the related art is explained in greater detail, referring to FIG. 22. FIG. 22 shows a cross-section of a HEMT of the related art, disclosed in this reference.
The HEMT 100 comprises a sapphire substrate 102, buffer layer 104, electron transit layer 106, electron supply layer 108, source and drain electrodes 118 and 120, and gate electrode 114.
The sapphire substrate 102 is approximately 630 μm thick. The buffer layer 104 comprises AlN of thickness approximately 1 μm, and is grown epitaxially on the c-plane of the sapphire substrate 102.
The electron transit layer 106 comprises undoped GaN of thickness approximately 2 μm, and is deposited on the buffer layer 104. In an arbitrary layer structure, “undoped” means “without the intentional introduction of impurities”. In the following explanation, when an “undoped” state is indicated, the abbreviation “UID-” (Un-Intentionally Doped) is added to the beginning of the layer structure's name.
The electron supply layer 108 comprises an AlN layer 110 and AlGaN layer 112, which are stacked in this order on the electron transit layer 106. Here, the AlN layer 110 is AlN of thickness approximately 1 nm.
The AlGaN layer 112 comprises first, second, and third AlGaN layers 112a, 112b and 112c, stacked in this order on the AlN layer 110. The first AlGaN layer 112a comprises UID-Al0.26Ga0.74N of thickness approximately 7 nm. The second AlGaN layer 112b comprises n-Al0.26Ga0.74N, of conduction type n, formed by Si doping at approximately 5×1018/cm3, and is approximately 15 nm thick. The third AlGaN layer 112c comprises UID-Al0.26Ga0.74N of thickness approximately 3 nm.
On the side of the electron transit layer 106 at the heterointerface 115 between the electron transit layer 106 and the electron supply layer 108, a two-dimensional electron layer 116 is formed. This two-dimensional electron layer 116 extends over a thickness of approximately 10 nm from the heterointerface 115, and is formed by an induced two-dimensional electron gas.
On both sides of the region of formation of this HEMT 100, isolation layers 124, 124 are formed and separated each other. The isolation layer 124 is provided to electrically separate a HEMT 100 from other adjacent elements of a chip, and are formed surrounding the region of formation of the HEMT 100.
The isolation layer 124 is electrically insulating, and extends from the top face 108a of the electron supply layer 108 to a depth exceeding the depth of the two-dimensional electron layer 116. The isolation layer 124 is formed by ion implantation.
Between the two isolation layers 124, 124 are provided source and drain electrodes 118, 120, at an interval from the isolation layers 124, 124. The source and drain electrodes 118, 120 are formed at a distance from each other. Source and drain electrodes 118, 120 are electrodes in Ohmic contact with the electron transit layer 106.
Below, the source and drain electrodes 118 and 120 are together called electrodes 128.
Recesses 126, which are depression formed in advance to a prescribed depth from the top face 108a of the electron supply layer 108 in the regions of formation of electrodes 128, is formed. The electrodes 128 are then formed so as to bury these recesses 126.
Between the source and drain electrodes 118, 120 is provided a gate electrode 114, connected by a Schottky junction to the electron supply layer 108.
FIG. 23 shows the relation between the depth of the recesses 126 (depth of the electrodes 128) and the contact resistance in the technology of the related art. FIG. 23 is a citation of FIG. 6 in the above reference.
In FIG. 23, the horizontal axis indicates the etching time (in minutes) required to form the recess 126, and corresponds to the depth of the recess 126. The vertical axis indicates the contact resistance (Ω-mm) between the electron transit layer 106 and the electrode 128. The arrow attached to the horizontal axis denotes the etching time corresponding to the depth of the heterointerface 115.
According to FIG. 23, in the region in which the depth of the electrode 128 is more shallow than the heterointerface 115, the contact resistance decreases with increasing depth.
However, when the depth of the electrode 128 becomes deeper than the heterointerface 115, the contact resistance increases.
From these results, it has been thought by person skilled in the art that, as the electrode 128 becomes deeper than the heterointerface 115, the contact resistance increases.